The invention relates to a method for determining supply voltages of a load, and also a load.
In this regard, the load is a multiphase load, that is to say a load that is connected to a multiphase, in particular three-phase, supply network. In this regard, the supply network comprises a plurality of supply phases and also additionally, or respectively an additional, neutral conductor phase.
In the context of monitoring systems, for example for monitoring the condition of the load or also for monitoring energy consumption, electrical variables of the load are determined during operation. Parameters for maintenance or wear are then derived from these variables, for example in the context of condition monitoring. A fundamental variable in this regard is the supply voltage for the load. In the case of a three-phase supply network, the individual supply voltages of a respective phase typically have a phase relationship of 120°. The load usually comprises a plurality of sub loads that are suitably connected to the individual phases. The load is a three-phase electric motor for example, and the sub loads individual coils or groups of coils in that electric motor.
Measuring the voltage of loads of this type requires that the supply voltages of the individual phases be determined. To this effect, a voltage divider is usually assigned to each phase for the purpose of measuring voltage, the voltage dividers being wired up to a common measurement potential as a reference point.
In real-life systems, however, and depending on the current operating situations, the measured voltage of a respective phase measured in this way is not necessarily identical to the actual supply voltage applied to the (sub) load. Thus, for example, the respective supply voltage is also dependent, inter alia, on how and in what way the load is wired up to the supply network, whether this is by way of a so-called star connection or by way of a so-called delta connection, for example. In the first case, a voltage at the level of the voltage of the respective supply phase applies in each case, usually 230 Volt. In the case of a delta connection, a higher voltage, typically around 400 Volt, is applied to the sub load. At the same time, however, a measured voltage of only 230 Volt would be measured in each case.
In the presence of unsymmetrical loading of the various phases, this can moreover also result in shifts in the phase angle, likewise resulting in differences being produced. Ultimately, potential differences frequently exist between the measurement potential, that is to say the reference potential for the measured voltage and the network potential, that is to say the reference potential for the individual (sub) loads.
In addition to the three conductor phases of a three-phase supply network, an additional neutral conductor is also connected to the load in some systems, which in that case defines the network potential, that is to say the reference potential.
There are also characteristic differences in the definition of network reference potentials according to the country. In the USA, for example, the potential of one of the supply phases is also used as a reference potential.
Nowadays, so-called voltage transformers are employed to even out potential differences, which transformers are connected in series with the measuring system as hardware components to prevent incorrect measurements due to potential differences. Voltage transformers of this type are employed for measuring power in external measuring modules, for example. However, this is associated with a high level of cost in terms of hardware.